tin

tin,

metallic chemical element; symbol Sn [Lat. stannum]; at. no. 50; at. wt. 118.710; m.p. 231.9681&degC;; b.p. 2,270&degC;; sp. gr. 5.75 (gray), 7.3 (white); valence +2 or +4. Tin exhibits allotropyallotropy[Gr.,=other form]. A chemical element is said to exhibit allotropy when it occurs in two or more forms in the same physical state; the forms are called allotropes......Click the link for more information.; above 13.2&degC; it is a lustrous, silver-white, highly crystalline metal with tetragonal structure. A brittle form with orthorhombic structure may exist above 161&degC;. Below 13.2&degC; pure tin tends to become a gray powder, a change commonly designated "tin pest" or "tin disease." Tin is very soft (only slightly harder than lead) and malleable; it can be rolled, pressed, or hammered into extremely thin sheets (tin foil). When iron or steel is dipped into molten tin, a layer of tin is deposited on the surface. A tin coating may also be applied by electroplating, which uses less tin. The tin serves to prevent rusting, since it is barely affected by moisture. The tin plate used in tin cans is an iron or steel sheet coated with tin. A tin coating is used to protect copper and other metals. Tin is a component of antifriction metal, bell metal, britannia metal, bronze, gunmetal, pewter, solder, and other alloys. Tin forms stannous compounds, in which it has valence +2, and stannic compounds, in which it has valence +4, as well as stannites, stannates, and other complex salts. Industrially useful compounds of tin include stannous chloride, important as a reducing agent, as a mordant in dyeing, and for weighting silk; stannic chloride, for the last two purposes and to stabilize perfume and color in soap; stannic oxide, for the preparation of white porcelain enamelware; and sodium stannite, a reducing agent. Stannous fluoride is added to toothpastes and water supplies to prevent tooth decay. Tin forms a number of toxic organometallic compounds that are used as fungicides, catalysts, and for other uses. Tin very rarely occurs uncombined in nature; the dioxide, which occurs as cassiterite, or tinstone, is the only ore of commercial importance. It is obtained chiefly from Bolivia, Indonesia, the Malay Peninsula, Congo (Kinshasa), and Nigeria. The tin mines of Cornwall, England, were formerly the principal source. The metal is prepared from cassiterite by heating in the reverberatory furnace. The ore from the mines is first given special treatment, and the "concentrates" thus obtained are mixed with coal in the furnaces. Tin was known and used by humans at least as early as the Bronze Age. The metal and its compounds were known and used by the alchemists. In 1673, Robert Boyle published a description of experiments on the oxidation (calcination) of tin. The metal was recognized as an element by Lavoisier.

tin

A lustrous white, soft, and malleable metal having a low melting point; relatively unaffected by exposure to air; used for making alloys and solder, and in coating sheet metal. See also: Metal

Tin

(Latin, stannum), Sn, a chemical element in group IV of the Mendeleev periodic table. Atomic number, 50; atomic weight, 118.69; a white, shiny, heavy, soft, and malleable metal. The element has ten naturally occurring isotopes: 112Sn, 114Sn, 115Sn, 116Sn, 117Sn, 118Sn, 119Sn, 120Sn, 122Sn, and 124Sn. 124Sn is slightly radioactive, and 120Sn has the largest natural distribution, with approximately 33 percent off all tin occurring in this form.

Historical information. Bronze, an alloy of tin and copper, was known as early as 4000 B.C., and pure tin was in use by 2000 B.C. Ancient civilizations fashioned ornaments, dishes, and utensils from tin. The precise origin of the words “stannum” and “tin” has not been established as yet.

Occurrence in nature. Tin is a basic component of the earth’s upper crust, comprising 2.5 × 10-4 percent by weight of the lithosphere, 3 × 10-4 percent of acid igneous rocks, and 1.5 × 10-4 percent of intrusive basic rocks. The mantle contains even smaller amounts of tin. Tin deposits are associated with magmatic processes (such formations as stanniferous granites and pegmatites) and with hydrothermal processes. Of the 24 stanniferous minerals, 23 were formed under conditions of high temperature and pressure. After cassiterite (SnO2), stannite (Cu2FFeSnS4) is today’s most commercially valuable mineral source of tin. Tin migrates poorly in the biosphere. Seawater contains only 3 × 10-7 percent tin, although certain species of seaweed exhibit a higher tin content. Nevertheless, tin does tend to diffuse throughout the biosphere.

Physical and chemical properties. Two allotropic forms of tin are known: α-Sn, or gray tin, and the more common β-Sn, or white tin. Gray tin occurs below 13.2°C and has a stable cubic lattice structure similar to that of diamond and a density of 5.85 g/cm3. White tin crystals are tetragonal. with lattice indexes a = 5.813 angstroms (Å) and c = 3.176 Å; the density of white tin is 7.29 g/cm3. The transformation of white to gray tin is accompanied by pulverization.

Bars of tin produce a characteristic crackling sound, called tin cry, when bent; this is a result of friction between crystalline layers. In accordance with the electron configuration of the outer shell (5s25 p2), tin exists in the oxidation states +2 and +4, the latter being more stable. Sn (II) compounds are strong reducing agents. The formation of a thin, strong, nonporous SnO2 film on the surface of tin upon exposure to dry or humid air practically prevents oxidation at temperatures of up to 100°C.

Tin remains stable in both cold and boiling water. The standard electrode potential of tin in acid medium is – 0.136 volts. Tin gradually displaces hydrogen in cold, dilute HCl and H2SO4 to form SnCl2 and SnSO4, respectively. It dissolves upon heating in hot, concentrated H2SO4 to yield Sn(SO4)2 and SO2. Dilute nitric acid at 0°C reacts with tin according to the equation

4Sn + 10HNO3 = 4Sn(NO3)2 + NH4NO3

Tin oxidizes upon heating with concentrated HNO3 (density 1.2–1.42 g/cm3) to produce a metastannic acid residue with a variable degree of hydration:

Atmospheric oxygen produces an SnO2 film on the surface of the metal. Stannic oxide (SnO2) is highly stable, but stannous oxide (SnO) undergoes rapid oxidation and therefore must be prepared by indirect methods. SnO2 displays predominantly acidic properties, and SnO basic properties.

Tin does not combine directly with hydrogen. For example, stannic hydride, SnH4, is formed upon interaction of Mg2Sn and hydrochloric acid:

Mg2Sn + 4HCl = 2MgCl2 + SnH4

The hydride SnH4 is a colorless, poisonous gas with a boiling point of – 52°C. Extremely unstable, SnH4 decomposes into Sn and H2 within a few days at room temperature; decomposition is instantaneous at temperatures above 150°C. SnH4 is also formed during the precipitation of Sn salts in the presence of hydrogen, for example,

SnCl2 + 4HCl + 3 Mg = 3MgCl2 + SnH4

Tin combines with halogens to yield compounds of the formulas SnX2 and SnX4. The former are saltlike compounds that yield Sn2+ ions in solution, while the latter (except SnF4) are both hydrolyzable and soluble in nonpolar organic liquids. The interaction of tin and dry chlorine yields stannic chloride (SnCl4), a colorless liquid that is used as a solvent for sulfur, phosphorus, and iodine. The reaction is

Sn + 2Cl2 = SnCl4

This reaction was previously employed to extract tin from defective tin-plated products. It is not widely used today, because of the chlorine toxicity and tin losses involved.

Tetrahalides of the formula SnX4 produce complexes with H2O, NH3, nitrogen oxides, and PCl5, as well as with alcohols, ethers, and many other organic compounds. Hydrohalide acids combine with tin halides to yield complex acids that are stable in solution, for example, H2SnCl4 and H2SnCl6. Simple or complex chloride solutions hydrolyze upon dilution with water or upon neutralization to form white Sn(OH)2 or H2SnO3 · nH2O precipitates. The interaction of tin and sulfur yields brown stannous sulfide (SnS) and golden yellow stannic sulfide (SnS2), both of which are insoluble in water and dilute acids.

Preparation and use. It is commercially profitable to extract tin from placer deposits with a 0.01 percent Sn content and from native deposits with a 0.1 percent Sn content; most deposits contain tenths of a percent or only a few percentage units of tin. In ores, tin is often associated with W, Zr, Cs, Rb, and rare-earth elements, as well as with Ta, Nb, and other precious metals. The raw material from placer deposits is usually enriched by gravitation; table flotation or simple flotation is usually used with ores from native deposits.

Concentrates, which contain from 50 to 70 percent tin, are roasted to facilitate sulfur extraction and treated with HCI to eliminate iron. If wolframite [(Fe, Mn)WO4] and scheelite (CaWO4) impurities are present, the concentrate is subjected to HCI treatment, and the resulting WO3 · H2O is removed with NH4OH. The concentrates are then smelted with coal in electric or combustion furnaces to produce crude tin, which is 94–98 percent tin and contains Cu, Pb, Fe, As, Sb, and Bi. After smelting, the crude tin is filtered through coke at 500°–600°C or centrifuged to separate most of the iron from the mix. The addition of elemental sulfur to the liquid metal removes any remaining Fe and Cu impurities, which float up as solid sulfides and are subsequently removed from the surface of the tin. A similar refining process is employed to remove arsenic and antimony (in which case the additive is aluminum) or lead (with the additive SnCl2). Sometimes Bi and Pb are evaporated under a vacuum. Electrolytic refining and recrystallization are only used in certain rare cases where especially pure tin is required.

Approximately 50 percent of all the tin that is produced is secondary metal, which is prepared from the waste products of tin-plating, from scrap metal, and from various alloys. Up to 40 percent is used for the tin-plating of food cans, and the remainder is used to manufacture solders, bearing alloys, and type metals. Stannic oxide serves in the preparation of heat-resistant enamels and glazes. Sodium stannite (Na2SnO3 · 3H2O) is used for the mordant dyeing of textiles. Crystalline SnS2, or gold plate, is combined with paints and applied to produce a gilded effect. Niobium stannide (Nb3Sn) is a very popular superconductor.

N. N. SEVRIUKOV

Toxicity. Tin, either alone or in combination with most inorganic substances, exhibits relatively low toxicity. Very few cases of acute poisoning have been induced by the wide use of elemental tin in industry. According to current scientific literature, individual cases of tin poisoning are apparently caused by the liberation of AsH3 when water accidentally falls on the waste material that forms during the removal of arsenic impurities from tin. Workers in tin-smelting plants may develop symptoms of pneumoconiosis from prolonged exposure to stannous oxide dust, or “black tin.” Cases of chronic eczema are sometimes known to occur among workers who prepare tin foil. Atmospheric concentrations of tin tetrachloride (SnCl4 · 5H2O) that exceed 90 mg/m3 irritate the upper respiratory tract and induce coughing, and tin chloride causes ulceration upon contact with skin. Stannic hydride (SnH4) is a strong convulsive poison, but the chances of its formation under industrial conditions are very slight. Severe poisoning after ingestion of canned foods that have been subjected to prolonged storage may be related to the formation of SnH4 in the containers, which is the result of the behavior of organic acids in the food during tinning. Acute SnH4 poisoning induces convulsions, equilibrium disorders, and even death.

Organic tin compounds, particularly dialkyls and trialkyls, exhibit a marked effect on the central nervous system. Symptoms of trialkyl poisoning include headache, vomiting, dizziness, convulsions, paresis, paralysis, and disturbances in vision; these are frequently followed by coma and fatal respiratory and cardiac failures. Dialkyl tin compounds exhibit slightly lower toxicity, and the poisoning is characterized by disorders of the liver and bile ducts. Industrial hygiene regulations should be followed to prevent tin poisoning.

Tin as an artistic material. Tin has been used in the decorative and applied arts because of its excellent casting properties, malleability, suitability for cutting, and brilliant silver-white color. The ancient Egyptians soldered tin onto various metals for ornamental purposes. Toward the end of the 13th century, Western Europeans began to fashion tin dishes and church wares that resembled silver articles but had smoother contours and deeper, more rounded lines in the etchings used for inscriptions and ornamentation. In the 16th century, F. Briot in France and C. Enderlein in Germany were the first to mold ornamental cups, plates, and goblets from tin; the relief figures were in the form of crests and scenes from mythology and daily life.

A. Sh. Bul’ used tin as a decorative trim on furniture. Tin mirror frames and utensils enjoyed widespread popularity in 17th-century Russia. The manufacture of copper trays, kettles, and snuffboxes trimmed with tin inlay and enamel flourished in Northern Russia during the 18th century. Toward the beginning of the 19th century, tin wares were gradually replaced by faience, resulting in the near disappearance of tin from the decorative-art market and the increasingly rare use of the metal for ornamental purposes. The high aesthetic value of modern tin ornaments stems from the sharp outlines of the shapes in objects and from the mirror finish, which is obtained by casting without subsequent processing.

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